Tne Federal Aviation Administration has certain minimum standards with respect to the light shape and intensity provided by elevated airport runway, taxiway, and threshold edge lights. For example, for blue taxiway lights, FAA Advisory Circular AC 150/5345-48 in FIG. 6 provides for an isocandela curve for minimum output in blue light for the L-861-T Taxiway light.
Similar standards are provided by the Federal Aviation Administration for other colored and white lights. Comparable standards exist in other countries dictated by the appropriate agency having jurisdiction in that particular country.
In order to meet these standards, the glass dome which surrounds the light source is formed with a varying internal shape which constitutes a prismatic lens so that the light may conform to these standards. It is essential that the relative relationship between the light source, which is an incandescent electric bulb, and the prismatic lens be maintained within very close tolerances. Variations of as little as 0.025 inches can result in the spectral shape not conforming to the minimum FAA or other country's requirements.
Additionally, the elevated airport runway, taxiway, and threshold edge lights must be capable of being positioned absolutely level even though the vertical post on which they are supported may not be perfectly vertical as the result of installation procedures. Even though there are slight variations in the elevation of the terrain on which the lights are positioned, it is essential that the lights all be in the same horizontal plane.
Furthermore, in climates where snow must be removed from the runway with snowplows, it is essential that the lights be identified with flags attached thereto which will extend above the snow so that the snowplow does not come in contact with the lights.
The present state of the art involves two types of elevated airport runway, taxiway, or threshold edge lights and are referred to as the twist lock tab- and the metal band- or metal clamp-type.
In the twist lock tab-type light the glass dome is provided with a segmented lip at the base thereof extending outwardly at right angles to the axis of the light in the form of three separate tabs. These tabs fit under comparable lips in the base of the light and the glass dome is secured by twisting it in a bayonet-type lock. The glass dome also abuts against an O-ring positioned in a groove in the upper periphery of the base as a seal against water, ice, and snow.
The disadvantages of this type of light are as follows: In the glass molding process, the thickness of the tabs at the base of the glass dome can vary as much as 1/16th of an inch in thickness. Thus when the glass dome is positioned in its bayonet joint the relative position of the prism in the glass dome with respect to the light source can vary to such an extent that the spectral shape of the light is altered so as not to meet minimum agency standards. Furthermore, the rotating motion of the glass against metal in seating the glass dome, either during initial installation or when replacing the light source, inherently has the possibility of chipping or cracking the dome and this is accentuated if ice or snow is present.
This type of light also, because of its construction, tends to channel any moisture into the area where the glass dome attaches to the metal base so that if there is any possible entrance point for moisture, the design encourages moisture to enter at such point.
The elevated airport runway, taxiway, and threshold edge lights of this twist lock tab-type also are leveled by means of four (4) separate leveling screws which require multiple adjustments and consequent excessive labor charges to achieve a level position of the light. Furthermore, the flag holder is so shallow that when the flag shaft is inserted therein, unless the shaft is wired or otherwise tied to the supporting conduit beneath the light, the flag will not remain in an upright position.
The metal band- or metal clamp-type elevated airport runway, taxiway, or threshold edge light has all of the disadvantageous features described above with respect to the twist lock tab-type light.
In the metal band- or metal clamp-type light, the glass dome is positioned on the metal base by means of a circumferentially extending metal clamp or band which engages an outwardly extending lip on the metal base and a corresponding outwardly extending lip on the base of the glass dome. This type of junction inherently permits moisture to enter the interior of the glass dome and also variations in thickness of the glass lip which are normal in the glass manufacturing process will permit unacceptable positioning differences between the prism portion of the glass dome and the light source so as to cause the spectral shape of the light not to meet minimum standards. This type of light also has the same leveling and flag-holding disadvantages described above with respect to the twist lock tab-type light.